Loss of skeletal muscle mass with aging (sarcopenia) has the potential to adversely impact thermal homeostasis in hypothermic conditions. Epidemiological data and limited laboratory-based experiments support this contention, and the detrimental effects could be exerted through any of several plausible mechanisms. First, sarcopenia alters the thermal properties of the body as a passive system because it changes the water content, and thus specific heat, of muscle and adipose tissues. Secondly, age-related changes in muscle mass also affect the insulation provided by peripheral tissues, especially in the limbs. This results in an increased reliance on peripheral vasoconstriction to minimize heat loss, a reflex response which is already attenuated in older individuals (Kenney and Armstrong, 1996). Finally, basal and resting metabolic rates (and thus metabolic heat production) decrease 20% from age 30 to age 70 due in large part to the loss of active muscle mass. This decrement may be associated with a decreased capacity for metabolic (both shivering and nonshivering) thermogenesis. Thus, gradual loss of muscle mass in the elderly is associated with an increased risk of hypothermia during cold stress; however, these purported mechanistic effects have never been directly tested. This proposal outlines a pilot project which uses a standardized cold challenge (hypothermic skin clamp, HSC) to systematically lower core temperature by convective and conductive heat transfer. Male subjects over the age of 70 will be recruited into two experimental groups which have a similar subcutaneous fat thickness and body adiposity, but differ significantly in body weight and fat-free weight (FFW). In addition to core (esophageal and rectal) and mean skin temperatures, metabolic rate (VO2), whole-limb and limb skin blood flows heart rate, and mean arterial blood pressure will be monitored. In this age cohort, it is hypothesized that: i. a lower FFW will be associated with a greater rate of drop in core temperature during the HSC; ii. a lower FFW will be associated with an attenuated increase in metabolic rate; iii. the calculated effective tissue insulation (IT) will be lower in the older men with a lower FFW; and, iv intergroup differences in FFW will not affect reflex whole-forearm or forearm skin vasoconstriction.